The present invention relates, in general, to an article of footwear for running and walking, and more particularly, to an article of footwear wherein the contour of the outer sole structure produces an increase in stride length and wherein there is reduced shearing, joint and bone trauma, and reduced muscle and tendon strain associated with the initial portion of each foot contact with a ground surface.
The interaction of the article of footwear or shoe with a ground surface during the stance phase of a gait cycle may be discussed in terms of events and stages. "Initial contact" is when a portion of the outer sole contacts the ground surface after the entire shoe has advanced forward during swing phase. Initial contact creates substantial force on the foot along the area of ground impact. "Foot-flat" is defined as that point when virtually all of the outer sole substantially comes to rest on the ground surface. "Heel-off" is when the heel area of the outer sole begins its rise off the running surface. It is generally agreed that the push-off stage of stance phase is roughly associated with the period between heel-off and "toe-off". "Toe-off" is when the forefoot portion of the outer sole leaves the ground surface to begin the swing phase of a gait cycle. Stride length is the distance between the point of toe-off of one foot and the heel at initial contact of the other foot.
One of the significant problems with conventional running shoes is that the ground-engaging surface of the sole is essentially flat and terminates in a relatively sharp edge along both the lateral and heel borders of the sole in the region of initial contact. The added material (which is generally the thickness of the sole) between the ground surface and the runner's foot reduces stride length by prematurely ending foot flight and creates an artificial fulcrum and leverage which promotes an unstable landing and which causes the foot to pronate and plantar-flex abruptly between initial contact and foot-flat. The increased joint action or movement velocities and accelerations/decelerations cause significantly increased impact muscle strain, and loadings on bones and joints of the extremity, especially those of the foot and ankle.
Conventional running shoes provide a substantially uniform frictional interface between the runner's sock and the shoe insole which is not efficient in terms of stride length, shearing trauma on foot impact, and wasted energy during push-off. A uniform high friction interface results in tissue shear trauma to plantar areas of initial contact, while a uniform low friction interface results in the foot sliding backward in the shoe during push-off, thereby wasting energy.